The invention relates to optical pellicles for photomasks used in photolithographic manufacturing. In particular, the invention relates to optical pellicles that may be attached to a photomask without adhesive, and may be removed and reattached.
The manufacture of semiconductor devices typically involves applying a layer of a photosensitive substance (a photoresist) to the surface of a target wafer. The photoresist is exposed to light in a selected pattern using a photomask, and the photoresist is then developed to leave exposed regions of the wafer. Typically, the exposed regions are subsequently etched away or otherwise modified, and the residual photoresist is removed. The pattern of the photomask typically possesses extremely fine details, and the presence of even tiny particles on the surface of the photomask can interfere with the accurate reproduction of the pattern on the target wafer.
To minimize particulate contamination at the mask surface, optical pellicles have been developed that protect the photomask. An optical pellicle is a frame-mounted transparent membrane that is attached to the photomask surface, so that contaminating particles fall onto the pellicle membrane and not the surface of the photomask. The pellicle frame holds the pellicle membrane at a sufficient distance above the mask surface so that any particles that may fall upon the membrane lie outside the focal plane of the illuminating light, and so fail to interfere with the projected mask pattern. The use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and has become widespread in the industry.
However, constant demand for smaller, faster, and more powerful microprocessors has required the semiconductor industry to fabricate ever smaller and faster semiconductor circuits. Manufacturing techniques have advanced to the point that the size of the circuit being produced is effectively limited by the wavelength of light used in the photolithographic process, with shorter wavelength illumination permitting finer details in the resulting circuit structure. Thus, photolithography using 248 nm, 193 nm, and 157 nm (Deep Ultra-Violet, or DUV) illumination has become common, and even the use of 13.5 nm (Extreme Ultra-Violet, or EUV) illumination is known.
However, as the wavelength of the illuminating light decreases, the energy of that light increases. Many airborne organic compounds that were benign at longer wavelengths become photolytically activated when exposed to energetic ultra-violet illumination. For example, light with a wavelength of 248 nm reacts with most halogenated organic compounds, and may interact with some non-halogenated organic compounds. Light having a wavelength of 193 nm reacts readily with a wide range of organic airborne contaminants, and 157 nm light is efficiently absorbed by and generates reactions with even the moisture present in air. The reactive breakdown products of these reactions can interact with the mask pattern itself, resulting in the generation of a variety of defects.
Unfortunately, one source of organic contaminants is the optical pellicles themselves. As shown in FIG. 1, a typical optical pellicle includes a pellicle membrane 6 mounted to a pellicle frame 5. The pellicle frame is typically attached to photomask 2 using adhesive 3. The operational lifetime of the optical pellicle is typically shorter than the operational lifetime of a given photomask, and so the optical pellicle must occasionally be replaced. Unfortunately, removing the optical pellicle often requires a special tool, and may damage the photomask itself. Even more problematic, current methods and adhesives used to adhere optical pellicles to the surfaces of the photomasks serve to generate additional particulate contamination whenever the optical pellicle is removed.
The use of adhesive to mount optical pellicles contributes a source of potential contamination (adhesive residue), and is poorly suited for the rapid removal and replacement required by modern semiconductor manufacturing techniques. What is needed is an optical pellicle that can be readily removed when desired, readily reattached to the photomask, does not leave adhesive residue on the photomask itself, and could be adapted for use by automated methods.
The invention is an optical pellicle, including a pellicle frame having two sides, a pellicle membrane mounted across the first side of the pellicle frame, and at least one non-adhesive mounting member configured to reversibly affix the second side of the optical pellicle to a photomask. The mounting member is optionally a vacuum mounting member, or an electrostatic mounting member. The optical pellicle of the invention is readily removed from the photomask and reattached, and is well-suited for use by automated methods.